In recent years, a power steering apparatus having a mechanism for providing assist force to steer the steering wheel has been widely employed in an automobile for the purpose of providing comfortable operability to a driver. In particular, in a light car or a compact car, there has been employed electric power steering in which rotation of an electric actuator for generating steering assist force is transmitted to wheels after the rotation is reduced through a predetermined gear and the like. Here, in such an electric power steering apparatus, one known as a resin gear, having a structure in which a synthetic resin gear portion is mounted on an outer peripheral surface of a metal sleeve or the like as a hub portion, has been generally employed (particularly as a driven gear) for transmitting the rotation of the electric actuator to the wheel. Employment of such a resin gear can reduce the weight and noise of the electric power steering apparatus, and also advantageously realizes low-fuel consumption of the light car or the like.
On the other hand, in a large car whose weight reaches as high as about 2 tons, a hydraulic power steering apparatus using engine power has been generally employed as a steering assist force. However, from the viewpoint of environmental problems, it has recently been demanded to reduce fuel consumption of vehicles. Accordingly, also in such a large car, it has been desired to employ an electric power steering apparatus largely contributable to reduction of fuel consumption thereof.
However, when the electric power steering apparatus is mounted on the large car, an electric actuator having a higher power than the light car or the compact car is required as the electric actuator thereof. However, a conventional resin gear has not had strength or durability sufficient to resist a high-power electric actuator.
Hence, there have been conducted studies and developments of such a resin gear for an electric power steering device having excellent mechanical strength and durability which can be advantageously used also in the electric power steering device equipped with the high-power electric actuator. For example, one of the inventors of this application has previously proposed a resin gear (worm wheel) formed with a thermosetting synthetic resin filled with an aramid resin and an electric steering device equipped therewith, in patent document 1 (U.S. Pat. No. 6, 557,663).
However, although some degree of improvement in mechanical strength, durability and the like is observed in the resin gear (worm wheel) proposed in the patent document 1, it has not been sufficient yet. Therefore, in the present circumstance, it has been strongly desired to develop a resin gear for an electric power steering apparatus which can exhibit more excellent mechanical strength and durability.
Patent Document 1: U.S. Pat. No. 6, 557,663